probplot <- function(x, qdist=qnorm, probs=NULL, line=TRUE,
                     xlab=NULL, ylab="Probability in %", ...)
{
    DOTARGS <- as.list(substitute(list(...)))[-1]
    DOTARGS <- paste(names(DOTARGS), DOTARGS, sep="=",
                     collapse=", ")

    xlab=deparse(substitute(x))

    x <- sort(x)
    QNAME <- deparse(substitute(qdist))
    DOTS <- list(...)
    qdist <- match.fun(qdist)
    QFUN <- function(p){
        args=DOTS
        args$p=p
        do.call("qdist", args)
    }
    
    y <- QFUN(ppoints(length(x)))

    if(is.null(probs)){
        probs <- c(.01, .05, seq(.1,.9, by=.1), .95, .99)
        if(length(x)>=1000)
            probs <- c(0.001, probs, .999)
    }

    qprobs <- QFUN(probs)
      
    plot(x, y, axes=FALSE, type="n", ylim=range(c(y,qprobs)),
         xlab=xlab, ylab=ylab)
    box()
    
    abline(h=qprobs, col="grey")
    axis(1)
    axis(2, at=qprobs, labels=100*probs)

    points(x, y)

    QTEXT <- paste("Quantile: ", QNAME, sep="")
    if(nchar(DOTARGS))
        QTEXT <- paste(QTEXT, DOTARGS, sep=", ")
    mtext(QTEXT, side=1, line=3, adj=1)
    
    xl <- quantile(x, c(0.25, 0.75))
    yl <- qdist(c(0.25, 0.75), ...)
    slope <- diff(yl)/diff(xl)
    int <- yl[1] - slope * xl[1]
    
    if(line){
        abline(int, slope, col="red")
    }
    z <- list(qdist=QFUN, int=int, slope=slope)
    class(z) <- "probplot"
    invisible(z)
}

lines.probplot <- function(x, h=NULL, v=NULL, bend=FALSE, ...)
{
    if(is.null(h) & is.null(v)){
        abline(x$int, x$slope, ...)
    }

    pu <- par("usr")

    if(!is.null(h)){
        h <- x$qdist(h)
        if(!bend){
            abline(h=h, ...)
        }
        else{
            v <- c(v, (h-x$int)/x$slope)
        }
    }

    if(!is.null(v)){
        if(!bend){
            abline(v=v, ...)
        }
        else{
            h <- v*x$slope+x$int
            segments(v, pu[3], v, h, ...)
            segments(pu[1], h, v, h, ...)
        }
    }
}
